We report a discovery of $z = 4.72$ radio galaxy, HSC J083913.17+011308.1, by using the Lyman break technique with the Hyper Suprime-Cam Subaru Strategic Survey (HSC-SSP) catalog for VLA FIRST radio sources. The number of known high-$z$ radio galaxies (HzRGs) at $z > 3$ is quite small to constrain the evolution of HzRGs so far. The deep and wide-area optical survey by HSC-SSP enables us to apply the Lyman break technique to a large search for HzRGs. For an HzRG candidate among pre-selected $r$-band dropouts with a radio detection, a follow-up optical spectroscopy with GMOS/Gemini has been performed. The obtained spectrum presents a clear Ly$\alpha$ emission line redshifted to $z=4.72$. The SED fitting analysis with the rest-frame UV and optical photometries suggests the massive nature of this HzRG with $\log{M_*/M_{\odot}} = 11.4$. The small equivalent width of Ly$\alpha$ and the moderately red UV colors indicate its dusty host galaxy, implying a chemically evolved and dusty system. The radio spectral index does not meet a criterion for an ultra-steep spectrum: $\alpha^{325}_{1400}$ of $-1.1$ and $\alpha^{150}_{1400}$ of $-0.9$, demonstrating that the HSC-SSP survey compensates for a sub-population of HzRGs which are missed in surveys focusing on an ultra-steep spectral index.

中文翻译：

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使用斯巴鲁 HSC (WERGS) 对射电星系进行广泛而深入的探索。三、使用莱曼突破技术发现 az = 4.72 射电星系

我们报告发现 $z = 4.72$ 射电星系 HSC J083913.17+011308.1，通过使用 Lyman 断裂技术和 Hyper Suprime-Cam Subaru 战略调查 (HSC-SSP) 目录，用于 VLA FIRST 射电源。到目前为止，$z > 3$ 的已知高 $z$ 射电星系 (HzRGs) 的数量非常少，无法限制 HzRGs 的演化。HSC-SSP 的深度和广域光学勘测使我们能够将莱曼断裂技术应用于对 HzRG 的大规模搜索。对于具有无线电检测的预选 $r$-band dropouts 中的 HzRG 候选者，已经执行了具有 GMOS/Gemini 的后续光谱学。获得的光谱呈现出清晰的 Ly$\alpha$ 发射线，红移至 $z=4.72$。使用静止帧 UV 和光学光度测量的 SED 拟合分析表明该 HzRG 具有巨大的性质，$\log{M_*/M_{\odot}} = 11.4$。Ly$\alpha$ 的小等效宽度和中等红色的 UV 颜色表明它的尘埃宿主星系，暗示一个化学演化的尘埃系统。无线电频谱指数不符合超陡频谱的标准：$\alpha^{325}_{1400}$ 为 $-1.1$ 和 $\alpha^{150}_{1400}$ 为 $-0.9 $，表明 HSC-SSP 调查补偿了在专注于超陡光谱指数的调查中遗漏的 HzRGs 亚群。